It is clear that part of the risk of developing breast cancer is genetically determined. The genetic contribution to breast cancer risk may arise from either common or rare variants (or both) in the population. In order to systematically assess both common and rare polymorphisms, we propose to apply a structured and highly multidisciplinary two pronged approach designed to initially maximize the power to detect the effect of each class of variants on breast cancer risk. For the common variants,we will focus on cancer association studies using DNA from breast cancer cases and controls in the Nurses' Health Study and breast cancer cases from the SPORE's High Risk Cohort. The rarer missense variants will be subjected to functional cellular and molecular assays, combined with tracking of the variants in cases of familial breast cancer. Any variant that scores positive (for function or association) will be pursued by the complementary approach. In the previous SPORE cycle, we employed a 3-dimensional (3D) culture model of breast acini to identifygenes from the 'Breast Cancer 1000' cDNA library that induce phenotypic changes resembling events associated with breast tumor initiation and progression. Building on these results, we will initially focus on four candidate receptor tyrosine kinases, which showed the most dramatic phenotypic effects: EGF-R, CSF-1R, ERBB2, anc IGF- 1R. This research approach will be extended to other genes in future years. In this proposal, we wilt specifically: 1) test common variants for association with breast cancer across a spectrumof risk classes, 2) test rare variants for distinguishable functional activity and association with familial and early-onset breast cancer, 3) examine the relationship between genetic variation and breast tumor receptor expression status in sporadic breast cancer and 4) when possible evaluate the association of circulating ligands for these receptors and subsequent breast cancer risk. Using this paradigm to investigate the role of genetic variation and breast cancer, we hope to identify modestly penetrant alleles conferring breast cancer risk